/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.heron.api.windowing.evictors;

import java.io.Serializable;
import java.util.concurrent.atomic.AtomicLong;

import org.apache.heron.api.Pair;
import org.apache.heron.api.windowing.Event;
import org.apache.heron.api.windowing.EvictionContext;
import org.apache.heron.api.windowing.EvictionPolicy;

/**
 * An eviction policy that tracks count based on watermark ts and
 * evicts events up to the watermark based on a threshold count.
 *
 * @param <T> the type of event tracked by this policy.
 */
public class WatermarkCountEvictionPolicy<T extends Serializable>
    implements EvictionPolicy<T, Pair<Long, Long>> {
  protected final int threshold;
  protected final AtomicLong currentCount;
  private EvictionContext context;

  private volatile long processed;

  public WatermarkCountEvictionPolicy(int count) {
    threshold = count;
    currentCount = new AtomicLong();
  }

  public EvictionPolicy.Action evict(Event<T> event) {
    if (getContext() == null) {
      //It is possible to get asked about eviction before we have a context, due to WindowManager
      // .compactWindow.
      //In this case we should hold on to all the events. When the first watermark is received,
      // the context will be set,
      //and the events will be reevaluated for eviction
      return Action.STOP;
    }

    Action action;
    if (event.getTimestamp() <= getContext().getReferenceTime() && processed < currentCount.get()) {
      action = doEvict(event);
      if (action == Action.PROCESS) {
        ++processed;
      }
    } else {
      action = Action.KEEP;
    }
    return action;
  }

  private Action doEvict(Event<T> event) {
        /*
         * atomically decrement the count if its greater than threshold and
         * return if the event should be evicted
         */
    while (true) {
      long curVal = currentCount.get();
      if (curVal > threshold) {
        if (currentCount.compareAndSet(curVal, curVal - 1)) {
          return Action.EXPIRE;
        }
      } else {
        break;
      }
    }
    return Action.PROCESS;
  }

  @Override
  public void track(Event<T> event) {
    // NOOP
  }

  @Override
  public EvictionContext getContext() {
    return context;
  }

  @Override
  public void setContext(EvictionContext context) {
    this.context = context;
    if (context.getCurrentCount() != null) {
      currentCount.set(context.getCurrentCount());
    } else {
      currentCount.set(processed + context.getSlidingCount());
    }
    processed = 0;
  }

  @Override
  public void reset() {
    processed = 0;
  }

  @Override
  public Pair<Long, Long> getState() {
    return Pair.of(currentCount.get(), processed);
  }

  @Override
  public void restoreState(Pair<Long, Long> state) {
    currentCount.set(state.getFirst());
    processed = state.getSecond();
  }

  @Override
  public String toString() {
    return "WatermarkCountEvictionPolicy{" + "} " + super.toString();
  }
}
